The invention relates to disc drives. More specifically, the invention relates to a method and apparatus for testing the static and dynamic characteristics of rotatable devices such as the pivot bearing assemblies which are used to support actuator arms. More specifically, the invention relates to testing the dynamic characteristics of desired portions of an actuator arm and pivot bearing assembly independently of the drive electronics of the disc drive.
Disc drives are used in workstations, laptops and personal computers to store large amounts of information in a readily accessible form. Typically, a disc drive includes a magnetic disc which is rotated at a constant high speed by a spindle motor. The disc surfaces are divided into a series of concentric data tracks. Each data track can store information as magnetic transitions on the disc surface.
A disc drive also includes a set of magnetic transducers that are used to either sense existing magnetic transitions during a read operation or to create new magnetic transitions during a write operation. Typically, each magnetic transducer is mounted in a head. Each head is mounted to a rotary actuator arm via a flexible element which can accommodate movement of the head during operation. The actuator arm serves to selectively position the head over a particular data track to either read data from the disc or to write data to the disc.
Typically, the actuator arm is driven by a voice coil motor. The magnetic transducers, mounted in heads, are present at the ends of the arms which extend radially outward from a substantially cylindrical actuator body. This actuator body is moveably supported by a ball bearing assembly known as a pivot bearing or pivot bearing assembly. The actuator body is parallel with the axis of rotation of the discs. The magnetic transducers, therefore, move in a plane parallel to the disc surface.
The voice coil motor typically includes a coil which is mounted in the actuator arm at the end opposite the heads. This coil is permanently immersed in a magnetic field resulting from an array of permanent magnets which are mounted to the disc drive housing. Application of DC current to the coil creates an electromagnetic field which interacts with the permanent magnetic field, causing the coil to move relative to the permanent magnets. The voice coil motor essentially converts electric current into mechanical torque. As the coil moves, the actuator arm also moves, causing the heads to move radially across the disc surface.
Control of this movement is accomplished via a closed loop servo system. In this control system, position (or servo) information is prerecorded on at least one surface of one of the discs. The servo system can be dedicated, which means that an entire disc surface is prerecorded with servo information. In this case, a particular head is dedicated to reading only servo information. Alternatively, the servo system can be embedded. This means that the servo information is interweaved with the user data, and is intermittently read by the same heads which are used to read and write information.
Servo system designers need to have an accurate picture of how the actuator moves. The mechanical static and dynamic characteristics of an actuator are directly related to servo performance. It is thus necessary to obtain this information for proper servo design. As the servo system attempts to position the actuator, the dynamic characteristics of the ball bearing can have an adverse effect on the accuracy of this positioning. Therefore, it is important to be able to determine the dynamic characteristics of the pivot bearing assembly and provide this information to the servo designers. This is especially important during track following mode.
Servo systems typically include two controllers, a seek controller and a tracking controller. The seek controller manages large head movements for approximate placement of the actuator arm. Then, the tracking controller is responsible for the small displacements necessary to follow a particular track. While under the control of the tracking controller, the pivot bearing which supports the actuator arm undergoes movements as small as less than about 0.003 degrees ball rotation and less than about 50 nanometers ball displacement. These movements are small enough to be adversely effected by the dynamics of ball movement.
It is thus desirable to be able to characterize both the static and dynamic characteristics of the pivot bearing in order to allow a servo designer to properly design the servo system.
Hysteresis is another problem for servo system designers. Hysteresis refers to the friction torque inherent in any rolling ball bearing device. A ball bearing will move in response to an input force. However, implementation of the same force in the opposite direction will fail to return the ball bearing to its exact starting position because of the hysteresis.
Until now, one test available for characterizing a pivot bearing assembly has been to measure its static torque. This has typically been done via a Bearing Static Torque Tester, such as the one manufactured by Measurement Research, Inc of San Fernando, Calif. This test is often performed by pivot bearing assembly manufacturers. One result of this test is to describe the friction of the ball bearings. Unfortunately, this information can not be accurately correlated to actual dynamic operating conditions of the actuator when under the control of a drive servo system.
Previous attempts at evaluating the dynamic characteristics of an actuator arm and pivot bearing assembly have been less than successful. These attempts have included generating and analyzing mechanical Bode plots. However, these tests were performed using the disc drive circuitry, i.e., the position error signal (PES) from the actuator assembly. This requires that for the disc drive, it had to be physically modified to gain access to this signal to allow testing. Specifically, the PES signal is tapped from the pre-amp chip present in the disc drive circuitry. Because the PES signal is used for testing, not only were the drive level servo electronics not eliminated, but the entire actuator arm, from head through pivot bearing, was being tested since there was no capacity to test particular components of the actuator assembly independently of either the drive electronics or the other components of the assembly.
Bode plots have been generated using laser Doppler vibrometers, however, information regarding the velocity of the actuator movement was not obtainable. In addition, tests using a laser Doppler vibrometer were conducted on the hard disc drive using the drive electronics which required physical modification of the drive itself as previously discussed.
Therefore, a need exists for a test system and apparatus that permits accurate characterization of the dynamic characteristics of components of an actuator assembly, such as pivot bearing assembly. A need exists for a test system and apparatus that allows particular components to be tested in isolation of other components of an actuator assembly such as a pivot bearing assembly. A need exists for a test system and apparatus which can characterize components of an actuator assembly such as a pivot bearing and actuator arm without requiring physical modification of the device being tested. Furthermore, a need exists for a testing apparatus and system that supports actuator assemblies with different operating characteristics to be tested.
Accordingly, the invention is found in an integrated test system which provides for determination of the dynamic characteristics of an actuator assembly. In a preferred embodiment of the invention, the integrated test system determines dynamic characteristics of an actuator assembly independently of the servo control electronics. In a preferred embodiment of the integrated test system of the present invention a desired component can be isolated and tested. In a preferred embodiment of the integrated test system of the present invention, no physical modifications to the device being tested are required.
Specifically, a preferred embodiment of the integrated test system of the present invention is found in a method of dynamically characterizing a desired component on an actuator assembly. The method includes mounting the actuator assembly on a test platform which has a motion sensor, a coil driver coupled to the actuator assembly, a microcontroller coupled to the motion sensor, the coil driver, a computer and a signal analyzer coupled to the coil driver, the motion sensor and the computer. The microcontroller receives displacement and velocity feedback signals from the motion sensor and is programmed to perform PID control based on the feedback signals. The method also includes inputting a command from the computer to conduct a position test on the desired component of the actuator assembly. The position test includes supplying a signal to the coil driver to cause movement of the desired component on the actuator assembly, aiming the motion sensor at the desired component on the actuator assembly, sensing displacement feedback from the desired component with the motion sensor, and collecting a current sense signal from the coil driver and displacement feedback from the motion sensor with the signal analyzer.
Another preferred embodiment of the integrated test system of the present invention is found in a method of calculating an inertia of a component. The method includes mounting the component on an actuator assembly located a test platform which has a motion sensor, a coil driver coupled to the actuator assembly, a microcontroller coupled to the motion sensor, the coil driver, a computer and a signal analyzer coupled to the coil driver, the motion sensor and the computer, wherein the microcontroller receives displacement and velocity feedback signals from the motion sensor and is programmed to perform PID control based on the feedback signals. The method further includes the step of inputting a command from the computer to conduct a position test on the desired component of the actuator assembly wherein the position test includes supplying a signal to the coil driver to cause movement of the actuator assembly, aiming the motion sensor at the component located on the actuator assembly, sensing displacement feedback from the component with the motion sensor, collecting a current sense signal from the coil driver and displacement feedback signals from the motion sensor, and calculating the moment of inertia of the component.
Another preferred embodiment of the present invention is found in an integrated test system for dynamically characterizing a desired component of an actuator assembly. The integrated test system includes a test platform suitable for mounting the actuator assembly to be tested, a motion sensor aimed at the desired component, a coil driver mounted on the test platform wherein the coil driver is operatively coupled to the actuator assembly to instruct the actuator assembly to move, a microcontroller mounted on the test platform, wherein the microcontroller is operatively coupled to the coil driver to exert PID control over the actuator assembly, and a signal analyzer mounted on the test platform, wherein the signal analyzer is operatively coupled to the motion sensor and the coil driver and the signal analyzer collects displacement and velocity feedback signals.
Another preferred embodiment of the present invention is found in an integrated test system for dynamically characterizing a desired component of an actuator assembly. The test system includes a test platform for mounting the actuator assembly, a motion sensor aimed at the desired component on the actuator assembly, means for operatively instructing the actuator assembly to move, for exerting PID control over the actuator assembly and collect put signal, and collect displacement and velocity feedback signals from the motion sensor.
These and other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.